Reprogramming of rye rDNA in triticale during microsporogenesis

Abstract

To test the hypothesis that interspecific genomic and chromosome interactions leading to nucleolar dominance could be reprogrammed in meiosis, we compared the expression of distinct nucleolar organizing region (NOR) loci in hexaploid triticale root tip meristematic cells, pollen mother cells and young pollen grains. Interphase and metaphase cells were silver stained to quantify nucleoli and active NOR loci respectively. A marked difference in the ribosomal RNA gene activity of each locus was observed when different types of cells were compared: in somatic and pollen mother cells, rRNA gene activity was mainly restricted to major wheat NORs (1B and 6B) with only a small contribution from rye NORs (1R). In contrast, in young pollen grains, all NORs present, including the 1R NORs, were consistently active. The expression of all NORs just after meiosis is considered to be a consequence of meiotic reprogramming of rye origin rDNA. Gene reprogramming mediated by the resetting of methylation patterns established early in embryogenesis is suggested to be responsible for the differential expression of the NORs of rye origin in distinct developmental stages of triticale.

Postembryonic Establishment of Megabase-Scale Gene Silencing in Nucleolar Dominance

“…In fully expanded rosette leaves of mature plants, A. thaliana -derived rRNA gene transcripts are undetectable, indicating that nucleolar dominance is complete in these leaves. Following bolting and flowering, low levels of A. thaliana -derived transcripts are again detected in the inflorescence tissue, consistent with prior results in Brassica and Triticale showing that nucleolar dominance is incomplete upon the transition to flowering [10] or gametogenesis [9], respectively. Collectively, these results suggest that nucleolar dominance is not evident in the major organs that developed during embryogenesis (cotyledons), is established progressively in organs that develop postembryonically from the shoot apical meristem, and is incomplete, or leaky, in reproductive tissues that arise late in development.…”

“…In hybrids within the plant genera Crepis and Triticea, underdominant NORs suppressed in vegetative tissues are active in male meiocytes [8], [9]. In Brassica allotetraploid hybrids, underdominant rRNA genes that are silenced in organs derived from the vegetative and inflorescence meristems (e.g.…”

Higher Levels of Organization in the Interphase Nucleus of Cycling and Differentiated Cells

“…2B). However, in haploid, dividing nuclei of germinating pollen, the 1R rDNA locus in a wheat background is decondensed and active (144), showing that the intranuclear organization and activity of the loci are dependent on the type of cycling cell.…”

Gene Expression and Nucleolar Dominance in Hexaploid Triticale and T. Aestivum

“…Rye carries a single pair of nucleolar organizing regions (NORs, the site of rRNA genes) on the 1R satellited chromosome, while tetraploid (A and B genomes) and hexaploid (A, B and D genomes) wheat have major rDNA loci on the short arms of satellited chromosomes 6B and 1B, and minor loci on non-satellited chromosomes, 5DS and 1AS, and additional probably normally unexpressed and perhaps variable loci, on 7DL, 5AL, 1BL, 7DS and 3DS (23,24). In the large majority of cells of the hybrids, the rye NOR is virtually undetectable by silver staining (6,20,30), a method that reveals interphase nucleoli and stains the metaphase NORs, which were transcribed during the previous interphase (14,15).…”